1. Field of the Invention
The present invention is generally directed to determining substructure permeability and, more particularly, to a system for and method of determining the permeability of substructure near a borehole by backscatter logging from microfractures.
2. Description of the Prior Art
Extractability of oil is significantly dependent on the permeability of the substructure to fluid flow. One characteristic of the formation which contributes to permeability is porosity. This characteristic and its suitability for oil extraction purposes presently can be assessed by a number of techniques, which include radioactivity, resistivity and acoustic velocity logging. In addition, much information is obtained by a detailed study of core samples, which have been removed during drilling of the borehole. However, porosity alone does not uniquely indicate the total permeability of the formation surrounding the borehole.
In low porosity formations, fractures may provide local short range drainage for the matrix and effective flow paths to the borehole. Even relatively small fractures can increase the total permeability greatly, without contributing much to the porosity of the formation. One way of assessing the contribution of large and small (micro) fractures to permeability is by examination of core samples, obtained in the hole-boring operation. However, such evaluations do not necessarily define adequately the permeability of the matrix, immediately surrounding the borehole, and are practically useless for indicating the permeability of the matrix, some distance away from the borehole. The ere drilling of the borehole may change the local stress patterns and thus alter or even hide fractures, already present in the core sample, or even introduce new ones. In addition, some fractures of interest may not intersect the core sample and hence cannot be found in the examination. Finally, core sampling is quite expensive.
Other methods of assessing permeability are used, such as flow testing the well. However, this technique is also expensive and time consuming as well, and therefore not attractive. Sonic and ultrasonic reflection detection techniques are also used. The borehole televiewer log is useful for "seeing" wall discontinuities, formation dip and fractures. Pulses of ultrasound are beamed to the walls of the borehole and received as reflections, as the tool rotates and simultaneously descends into the borehole at slow speed. An intensity-modulated display of the inner surface of the borehole is obtained, but this tool has little value for "seeing" fractures some distance away from the borehole into the formation.
Another tool that is used is the acoustic amplitude log which is a sonic reflection type of tool, similar to the velocity logging tool. Sound pulses are generated, which travel from the source to the receiver, located a few feet below, with the waves taking various paths in the borehole mud and through the adjoining formations. The intensity-modulated display of the amplitude of the received signal vs the time of travel indicates presence of large fractures as regions of low signal. To that extent, this tool is useful for making some assessment of permeability.
There is a need for better techniques and means for evaluating tight fracture patterns, at least several feet out into the formations surrounding the borehole and to assess their contribution to overall permeability of the matrix.